Mortise and Tenon question

[Woodenskye (Bryan)]

Thread transferred from previous forum - Original Post by Chet

Original Post:

I have a question for the group.

On another forum a member is working on a couple of mission style end tables and in the conversation he mentioned cutting his mortises with the router and squaring up the ends for the tenons. The member that is making the tables is somewhat new to the hobby and I thought I would share another way of doing the same thing that he might find faster. Instead of squaring up the mortise you can round over the tenons.

Here comes my question. Another member stated -

“Honestly rounding isn't worth the time, chop the tenon narrow enough to fit and leave the rounded mortise sections empty they don't contribute noteworthy strength to the glue up”.

This is the second time I have seen him say this, the first time I heard him say this he said that he came up with this theory because the domino has a setting for cutting wider slots. I thought the wide setting on the domino was to allow for movement in something like breadboard ends. It seems to me that leaving a void like that is going to make the joint weaker and in the case of when he is telling this to a newb, I would like to rebut, but I myself have no scientific or mathematical information to back my thinking up. I personally cut mortise and tenons three ways, I rout the mortise and square the ends with a chisel to fit the tenon, I rout the mortise and round the tenon to fit the mortise because this is a little faster and I use a domino at times because it is real fast. It all depends on the type of project and how I feel at the time. I use these methods because they were taught to me by people I considered more versed then me at the time of my education.

What say you.

Follow Up Posts:

1)  I have used your 3 methods, what you mentioned was posted doesn't seem to make sense. The only thing I could come up with is if the mortise is bigger than the tenon, the piece isn't connected and the glue will not have anything to bond to. This is a completely dumbed down example, but if you had a table with PVC pipe as the legs and a smaller pipe attached to the table top that slides into the legs, where is the connection? Maybe I'm not thinking clearly on this.

Edit:  after reading the thread I think you are talking about, I can see the point he is trying to make, but you will always end up with a tiny gap, will you see it, maybe not, but getting tight fitting joinery is a good feeling.

2)  The pvc is a good analogy but there is very little contact between the two as they are round. Chet, I’ve often wondered the the same, is is it worth it to round over/square the joints. Great question. Personal opinion without merit, I think if you have sufficient long grain to long grain contact, the minimal gap on the ends shouldn’t make a lot of difference. 

3)  From a strength standpoint, cutting the tenon short is probably fine. Those big flat surfaces for the glue is where the strength comes from. This is provided that you didn't cut the tenon too short where the joint could wiggle loose over time. Now, with all that said, I still prefer tight fitting joinery rather than doing a sloppy job especially on a client project!